1. Field of the Invention
The present invention relates to an organic electro-luminescence device, and more particularly to an organic electro-luminescence device that is adaptive for improving the reliability of an electro-luminescence cell, and a method and apparatus for driving the same.
2. Description of the Related Art
Recently, there have been developed various flat display devices, which can be reduced in weight and bulk where a cathode ray tube CRT has a disadvantage. Such flat display panel includes a liquid crystal display LCD, a field emission display FED, a plasma display panel PDP, and an electro-luminescence (EL) display device.
The structure and fabricating process of the PDP among these are relatively simple. A PDP is advantageous to be made light, thin and large-sized, but the light emission efficiency and brightness thereof is low and its power dissipation is high. It is difficult to make an active matrix LCD (where a thin film transistor TFT is used as a switching device) large-sized because of using a semiconductor process, but since it is mainly used as a display device of a notebook computer, the demand for it is increasing.
As compared with this, the EL device is generally classified into an inorganic EL device and an organic EL device in accordance with the material of a light-emission layer. The EL device being a self-luminous device has an advantage that its response speed is fast, its light-emission efficiency and brightness are high, and it has a wide viewing angle.
The organic EL device, as shown in FIG. 1, has an anode electrode 2 formed of a transparent electrode pattern on the glass substrate 1. There are deposited a hole injection layer 3, a light emission layer 4, an electron injection layer 5 on top of the anode electrode 2. There is formed a cathode electrode 6 of metal electrode on the electron injection layer 5.
If a drive voltage is applied to the anode electrode 2 and the cathode electrode 6, holes in the hole injection layer 3 and electrons in the electron injection layer 5 each progress toward the light emission layer 4 to excite the light emission layer 4, so the light emission layer 4 is caused to emit visible ray. In this way, a picture or an image is displayed with the visible ray emitted from the light emission layer 4.
Referring to FIG. 2, the organic electro-luminescence device includes m number of column lines CL1 to CLm, n number of row lines RL1 to RLn, and m×n number of pixels Pixels (7) arranged in a matrix having the column lines CL1 to CLm cross the row lines RL1 to RLn.
FIG. 3 shows a circuit diagram of each pixel 7 in the device of FIG. 2. As shown in FIG. 3, the organic EL device includes a first TFT T1 formed at each intersection area of the column lines CL1 to CLm and the row lines RL1 to RLn to act as a switching device, a second TFT T2 formed between a corresponding cell drive voltage source VDD and a corresponding electro-luminescence cell OELD for driving the corresponding electro-luminescence cell OELD, and a capacitor Cst connected between the first and second TFT's T1 and T2. The first and second TFT's T1 and T2 are p-type MOS-FETs.
The first TFT T1 is turned on in response to a negative scan voltage from a corresponding one of the row lines RL1 to RLn to make a current path conduct electricity between the source terminal and the drain terminal of itself, and is sustained at an off-state when the voltage in the corresponding one of the row lines RL1 to RLn is lower than its threshold voltage Vth. While the first TFT T1 remains at its on-state, the data voltage VCL from a corresponding column line CL is applied to the gate terminal of the second TFT T2 through the source terminal and the gate terminal of the first TFT T1. Contrary to this, the current path between the source terminal and the drain terminal of the first TFT T1 is open during the off period of the first TFT T1, so the data voltage VCL is not applied to the second TFT T2.
The second TFT T2 controls the current between the source terminal and the drain terminal in accordance with the data voltage VCL applied to the gate terminal of itself to cause the electro-luminescence cell OLED to emit light in a brightness corresponding to the data voltage VCL.
The capacitor Cst stores a difference voltage between the data voltage VCL and the cell drive voltage VDD to cause the voltage applied to the gate terminal of the second TFT T2 to be sustained uniformly for one frame period and at the same time to sustain the current applied to the electro-luminescence OLED uniformly for one frame period.
FIG. 4 represents a scan voltage and a data voltage applied to the organic electro-luminescence device shown in FIG. 2.
Referring to FIG. 4, the row lines RL1 to RLn are sequentially supplied with negative scan pulses SCAN, and the column lines CL1 to CLm are simultaneously supplied with data voltages DATA synchronized with the scan pulses SCAN. Because of this, the data voltage DATA flows through the first TFT T1, and the data voltages are charged in the capacitor Cst.
Further, in such a structure, there is required a number of column lines as many as pixel signals of RGB are inputted.
FIG. 5 is another equivalent circuit diagram of a pixel which may be used for each pixel 7 in the organic electro-luminescence device shown in FIG. 2.
Referring to FIGS. 2 and 5, the organic electro-luminescence device includes m number of column lines CL1 to CLm, n number of row lines RL1 to RLn, and m×n number of pixels Pixels (7) arranged in a matrix having the column lines CL1 to CLm cross the row lines RL1 to RLn.
Further, for each pixel 7, in this example, the organic EL device includes a first TFT T11 formed between the cell drive voltage source VDD and the electro-luminescence cell OLED to drive the electro-luminescence cell OLED; a second TFT T12 connected to the cell drive voltage source VDD to form a current mirror with the first TFT T11; a third TFT T13 connected to the second TFT T12, the corresponding column line CL and the corresponding row line RL to respond to the signal in the corresponding row line RL; a fourth TFT T14 connected between the gate terminals of the first TFT T11 and the second TFT T12, the row line RL and the third TFT T13; and a capacitor Cst connected between the gate terminals of the first TFT T11 and the second TFT T12 and the voltage supply line VDD. The first to fourth TFT's T11 to T14 are p-type MOS-FETs.
The third and fourth TFT's T13 and T14 are turned on in response to a negative scan voltage from the row line RL to make a current path conduct electricity between their source terminal and the drain terminal, and are sustained at an off-state when the voltage in the row line RL is lower than their threshold voltage Vth. While the third and fourth TFT's T13 and T14 remain at their on-state, the data voltage VCL from the corresponding column line CL is applied to the gate terminal of the first TFT T11 through the third and fourth TFT's T13 and T14. Contrary to this, the current paths between the source terminal and the drain terminal of the first and second TFT's T11 and T12 are open during the off-period of the first and second TFT's T11 and T12, so the data voltage VCL is not applied to the first TFT T11.
The first TFT T11 controls the current between its source and drain terminals in accordance with the data voltage VCL applied to its gate terminal to cause the electro-luminescence cell OLED to emit light in a brightness corresponding to the data voltage VCL.
The second TFT T12 is configured in a current mirror form with the first TFT T11 to control the current from the first TFT T11 uniformly.
The capacitor Cst stores a difference voltage between the data voltage VCL and the cell drive voltage VDD to cause the voltage applied to the gate terminal of the first TFT T11 to be sustained uniformly for one frame period and at the same time to sustain the current applied to the electro-luminescence OLED uniformly for one frame period.
FIG. 6 represents a scan voltage and a data voltage applied to the electro-luminescence device shown in FIG. 5.
Referring to FIG. 6, negative scan pulses SCAN are sequentially applied to the row lines RL1 to RLn and data voltages DATA synchronized with the scan pulses SCAN are simultaneously applied to the column lines CL1 to CLn. Because of this, the data voltages DATA are charged in the capacitor Cst through the third and fourth TFT's T13 and T14. The data voltage DATA charged in the capacitor Cst is held for one frame period, and then controls the current path of the first TFT T11. Further, in such a structure, there is required a number of column lines as many as pixel signals of RGB are inputted.
In the circuit diagrams as above, in case of FIG. 3, the second TFT T2 is driven by the cell drive voltage VDD, i.e., DC voltage, while the electro-luminescence cell OLED is turned on, differently from the first TFT T11. Further, in case of FIG. 5, the first TFT T11 is driven by the cell drive voltage VDD, i.e., DC voltage, while the electro-luminescence cell OLED is turned on, differently from the third and fourth TFT's T13 and T14.
As described above, the electro-luminescence cell OLED of the organic electro-luminescence device according to the related art is always connected to the ground GND. Thus the electro-luminescence cell OLED is driven only in a forward direction. Due to this limitation, residual currents (e.g., status charges) are accumulated within the electro-luminescence cell OLED when being driven for a long time. Such residential currents interference with the recombination process of the holes with the electrons in the light emission layer 4, whereby the lifetime, reliability and effectiveness of the organic EL device are significantly reduced.